As a new generation display technology, organic light-emitting diode (OLED) panels have the advantages of low power consumption, high brightness, high resolution, wide viewing angle, high response speed, and etc., and thus are quite popular to the market.
Based on the driving methods, OLED displays can be sorted as the passive matrix OLED (PMOLED) display and the active matrix OLED (AMOLED) display. The AMOLED display features the active driving part to drive the pixels arranged in a matrix, has the advantage of high illumination efficiency, and thus is usually used as a large-scale display with high resolution.
FIG. 1 is a circuit diagram of a conventional OLED 2TIC pixel driving circuit. As shown, the technology of the conventional driving method and the pixel structure thereof is to apply different DC driving voltages to the OLED to have the OLED generates the needed color and brightness in different grayscales. 2T1C refers to the usage of two transistors and one capacitor, wherein the transistor T2 is the switching TFT, which is controlled by a scan signal Gate, and is utilized for controlling the entry of a data signal Data and acts as a switch to control charge/discharge of the capacitor Cst. The other transistor T1 is the driving TFT, which is utilized for driving the OLED by controlling the current passing through the OLED. The capacitor Cst is mainly utilized for storing the data signal Data so as to control the driving current applied to the OLED through the transistor T1. As an example, in the circuit diagram shown in FIG. 1, both the TFTs T1 and T2 are P-type TFTs, the scan signal Gate may come from a gate driver corresponding to a specific scan line, and the data signal Data may come from a source driver corresponding to a specific data line. OVDD is a high voltage power source, and OVSS is a low voltage power source.
After the scan signal Gate turns on the switch, the voltage Vdata of the data signal Data would be applied to the driving TFT T1 and stored in the capacitor Cst to have the transistor T1 stays in the on-state. Thus, the OLED would be continuingly placed in the DC-biased state and the internal ions would be polarized to form the internal electric field, which may result in the increasing of threshold voltage of the OLED and the brightness of the OLED would be steadily declined. The continuingly illumination would reduce the lifespan of the OLED. In addition, different degradation of the OLED pixels would result in display non-uniformity which may affect the display quality.
FIG. 2a is a circuit diagram of a conventional OLED 5TIC pixel driving circuit. FIG. 2b is a timing diagram of the circuitry shown in FIG. 2a. As shown, the circuit includes five thin-film transistors T1-T5 and one capacitor Cs. As an example, all the TFTs are N-type TFTs, and the input signals include a data voltage Vdata, a scan signal SCAN, an illumination signal EM, a DC high voltage VDD, and a DC low voltage VSS. According to the timing diagram, the driving process of the OLED is controlled by the scan signal SCAN (specified as S1 and Sn in the timing diagram to represent the scan signals of column 1 and column n), the illumination signal EM, and the DC high voltage VDD and is divided into two stages, i.e. data storing and threshold compensation stage and illumination stage. However, the conventional OLED 5T1C pixel driving circuit has the following drawbacks: the voltage level of VDD needs to be changeed, the rapidly changeing voltage level and the large level difference may result in the insufficient charge/discharge time and the current may get too high; the hardware for changing the voltage level of VDD is complicated, and the driving transistor should be a P-type transistor in order to eliminate voltage drift.
In conclusion, each of the aforementioned conventional OLED pixel driving circuits has the drawbacks need to be resolved. As shown in FIG. 1, the driving method of the conventional OLED 2T1C pixel driving circuit may result in degradation of the OLED easily because the voltage Vdata would be stored in the capacitor Cst to have the driving TFT stays in the on-state after the scan signal Gate turns on the pixel driving circuit so as to have the OLED continuingly placed in the DC-biased state. As shown in FIG. 2a and FIG. 2b, the conventional OLED 5T1C pixel driving circuit cannot be accomplished without the operations to eliminate the threshold voltage and to change the voltage level of VDD.